The commercial water heater industry has been served by storage tank water heaters that are sized to contain sufficient water at a specified temperature to satisfy demand during the highest expected usage. While this method has proven satisfactory for most applications, it requires large storage volumes, with associated losses, large footprint, and excessive set point temperatures to ensure performance. The commercial water heaters are typically operated in various ways.
One way is to select a maximum and minimum temperature set point relatively far apart from one another in order to minimize the frequency of power cycling of the water heater. For example, the maximum temperature set point might be twenty degrees higher than the desired water temperature. The water heater is cycled on until the actual water temperature reaches the maximum temperature set point. When the actual temperature of water in the heater drops to the minimum temperature set point at around the desired temperature, power to the water heater is cycled on again until the actual temperature reaches the maximum temperature set point. A drawback with this approach is that an inordinate amount of energy is required for heating the water in the water heater to a temperature well in excess of the desired temperature. Moreover, the excessive temperature can lead to scalding should water be drawn toward the end of an operating cycle. Further, employing a large water heater can typically leads to temperature striations along various levels of the water heater leading to high fluctuations in water temperature should a high load demand be suddenly imposed on the water heater.
A second way to operate a large water heater is to select a maximum and minimum temperature set point relatively close to one another in order to minimize energy consumption. For example, the maximum temperature set point might be only a few degrees higher than the desired water temperature. The water heater is cycled on until the actual water temperature reaches the maximum temperature set point. When the actual temperature of water in the heater drops to the minimum temperature set point at around the desired temperature, power to the water heater is cycled on again until the actual temperature reaches the maximum temperature set point. A drawback with this approach is that the close proximity between the maximum and minimum temperature set points results in frequent on and off power cycling which can shorten the operating life of the equipment for cycling power to the water heater.
Instantaneous heaters have also been applied with limited success. Their inability to respond to instantaneous flow changes and high cycling rates of the water heater due to recirculation loads has limited use by this method.
Accordingly, it is a general object of the present invention to overcome the drawbacks associated with prior water heater systems.